Almost all physiological processes are based on molecular recognition of peptides or proteins and other biologically active components and the like. A lot of peptides having important biological functions such as hormones, enzymes, inhibitors, enzyme substrates, neurotransmitters, immunomodulators and the like have been found to date. There are resultantly many studies conducted to develop therapeutic means with a peptide, with understanding physiological effects of active substances composed of these peptides.
In development of a peptide as a medicinal product, there are new methods established for treatments and therapies of diseases correlated with peptides, however, in use of a peptide as a medicinal product, problems as described below are generated. That is, a) under physiological conditions, most peptides are decomposed by specific and nonspecific peptidases, to give low metabolic stability, b) due to relatively large molecular weight thereof, absorption after ingestion is poor, c) excretion through liver and kidney is fast, and d) since a peptide is structurally flexible and receptors for a peptide can be distributed widely in an organism, undesired side effects occur in non-targeted tissues and organs.
Except for some examples, relatively small natural peptides (peptide composed of 30 to less than 50 amino acids) are present under disorderly conditions due to a lot of conformations in dynamic equilibrium in a diluted aqueous solution, as a result, the peptides lack in selectivity for a receptor and become liable to undergo metabolism, thus, determination of a biologically active conformation is difficult. When a peptide itself has a biologically active conformation, namely, when having the same conformation as that under condition linked to a receptor, a reduction in entropy in linking to a receptor is smaller as compared with a flexible peptide, consequently, an increase in affinity to a receptor is expected. Therefore, there is a need for a biologically active peptide having a uniformly controlled conformation, and development thereof is important.
There are recently many efforts conducted to develop a peptide mimic or a peptide analog (hereinafter, referred to as “peptide mimic” together) showing a more preferable pharmacological property than that of a natural peptide as the original form thereof. “Peptide mimic” used in the present specification is a compound which is capable of mimicking (agonistic substance) or blocking (antagonistic substance), at receptor level, the biological effect of a peptide, as a ligand of a receptor. For obtaining a peptide mimic as the most possible agonistic substance, factors such as a) metabolic stability, b) excellent bioavailability, c) high receptor affinity and receptor selectivity, d) minimum side effects, and the like should be taken into consideration. From the pharmacological and medical standpoint, it is often desirable not only to mimic the effect of a peptide at receptor level (agonistic action) but also, if necessary, to block a receptor (antagonistic action). The same items as the pharmacological items which should be considered for designing a peptide mimic as the above-described agonistic substance can be applied also to designing of a peptide antagonistic substance.
One example of peptide mimics is development of a peptide having a controlled conformation. This mimics, as correctly as possible, a conformation linked to a receptor of an endogenic peptide ligand. When analogs of these types are investigated, resistance to a protease increases, and resultantly, metabolic stability rises and selectivity rises, thereby lowering side effects.
Overall control in the conformation of a peptide is possible by restricting flexibility of a peptide chain by cyclization. Cyclization of a biologically active peptide not only improves its metabolic stability and selectivity for a receptor but also gives a uniform conformation, thereby enabling analysis of the conformation of a peptide. The cyclization form is the same as that observed in natural cyclic peptides. Examples thereof include side chain-side chain cyclization, or side chain-end group cyclization. For cyclization, side chains of amino acids not correlated with receptor recognition can be mutually linked, or can be linked to the peptide main chain. As another embodiment, there is head to tail cyclization, and in this case, a completely cyclic peptide is obtained.
For these cyclization operations, a cross-linking technology is imperative. Typical examples of cyclization include cross-linkages via a disulfide bond (SS bond), an amide bond, a thioether bond and an olefin bond. More specific examples thereof include cyclization by connecting two penicillamine residues via a disulfide cross-linkage (Mosberg et al., P.N.A.S. US, 80:5871, 1983), cyclization by forming an amide bond between lysine and aspartic acid (Flora et al., Bioorg. Med. Chem. Lett. 15 (2005) 1065-1068), a procedure in which an amino acid derivative containing a cross-linked portion having a thioether bond introduced previously is introduced into a peptide bond and cyclization thereof is performed in the last condensation reaction (Melin et al., U.S. Pat. No. 6,143,722), and cyclization by mutually cross-linking (S)-α-2′-pentenylalanines introduced into the main chain using an olefin metathesis reaction (Schafmeister et al., J. Am. Chem. Soc., 122, 5891-5892, 2000).
A cross-linkage via a disulfide bond, however, will be cleaved by a reductase generally present in an organism. Also, a cross-linkage via an amide bond will be cleaved by an enzyme cutting an amide structure present in an organism. A thioether bond and an olefin bond need substitution of side chains of an amino acid in a peptide elongation process, for attaining cyclization thereof.
Also known is a cross-linked structure originating from nitrogen constituting an amide in the peptide main chain skeleton, as a method needing no modification of a side chain of a peptide (Gilon et al., Biopolymers 31:745, 1991). However, this peptide will be cleaved by an enzyme cutting an amide structure, because of inclusion of an amide bond in this peptide.
Further, known as a cross-linked peptide having a molecular structure capable of linking to other substituent is a cross-linked peptide utilizing 2,4,6-trichloro[1,3,5]-triazine (Scharn et al., J. Org. Chem. 2001, 66, 507-513). In this method, however, the reaction in forming a cross-linked portion is an aromatic nucleophilic substitution reaction, thereby limiting applicable peptides.
As the analogous peptide, a cross-linked peptide in which a side chain and a carboxy terminus are linked is known (Goodman et al., J. Org. Chem. 2002, 67, 8820-8826). This peptide, however, will be cleaved by an enzyme cutting an amide structure, because of inclusion of an amide bond in this peptide.
A peptide having a controlled conformation is expected to provide a lot of pharmacological use applications. For example, somatostatin is a cyclic tetradecapeptide present in both the central nerve system and surrounding tissues and has been identified as an important inhibitor against secretion of a grow hormone from pituitary gland, and additionally, has functions such as suppression of secretion of glucagon and insulin from spleen, regulation of most gastrointestinal hormones, regulation of release of other neurotransmitters correlated with motor activity and a recognition process all over the central nerve system, and the like. A cross-linked peptide composed of nine amino acids called a WP9QY (W9) peptide mimicking the steric structure of a contact site between TNF and a TNF receptor suppresses the inflammation activity of TNFa, and additionally, is known to suppress bone resorption (Aoki et al., J. Clin. Invest. 2006; 116(6):1525-1534).
There is a study conducted to obtain a peptide mimic having metabolic stability improved by adding to the peptide a structure not present in natural peptides, as the peptide mimic showing a more preferable pharmacological property than that of a natural peptide as the original form thereof, in addition to a cross-linked peptide having a conformation controlled as described above. For example, resistance to an enzyme is improved by using cross-linkages (a cross-linkage via a thioether bond, a cross-linkage via an olefin, and the like) other than the above-described natural cross-linking (disulfide cross-linkage). Further, resistance to metabolism in an organism is improved by adding, for example, PEG and the like, to the terminus or the side chain of a peptide.
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